106 ON THE ORIGIN OF SPECIES 



The accompanying- diagram will aid us in under- 

 standing this rather perplexing subject. Let A to L 

 represent the species of a genus large in its own 

 country ; these species are supposed to resemble each 

 other in unequal degrees, as is so generally the case 

 in nature, and as is represented in the diagram by 

 the letters standing at unequal distances. I have said 

 a large genus, because we have seen in the second 

 chapter, that on an average more of the species of 

 large genera vary than of small genera ; and the vary- 

 ing species of the large genera present a greater number 

 of varieties. We have, also, seen that the species, 

 which are the commonest and the most widely-diffused, 

 vary more than rare species with restricted ranges. 

 Let (A) be a common, widely-diffused, and varying 

 species, belonging to a genus large in its own country. 

 The little fan of diverging dotted lines of unequal 

 lengths proceeding from (A), may represent its varying 

 offspring. The variations are supposed to be extremely 

 slight, but of the most diversified nature ; they are 

 not supposed all to appear simultaneously, but often 

 after long intervals of time ; nor are they all supposed 

 to endure for equal periods. Only those variations 

 which are in some way profitable will be preserved 

 or naturally selected. And here the importance of 

 the principle of benefit being derived from divergence 

 of character comes in ; for this will generally lead to 

 the most different or divergent variations (represented 

 by the outer dotted lines) being preserved and accumu- 

 lated by natural selection. When a dotted line reaches 

 one of the horizontal lines, and is there marked by a 

 small numbered letter, a sufficient amount of variation 

 is supposed to have been accumulated to have formed 

 a fairly well-marked variety, such as would be thought 

 worthy of record in a systematic work. 



The intervals between the horizontal lines in the 

 diagram, may represent each a thousand generations ; 

 but it would have been better if each had represented 

 ten thousand generations. After a thousand genera- 

 tions, species (A) is supposed to have produced two 

 See diagram at the commencement of volume. 



